Rapid progress has been made towards elucidating the underlying mechanisms controlling flower development in distantly related dicot plant species 5, 8!. These studies led to the isolation of a family of genes which encode regulatory proteins. These include AGAMOUS (AG) 30!, APETALA1 (AP1) 15!, and APETALA3 (AP3) 11! in Arabidopsis thaliana, and DEFICIENS A (DEFA) 25!, GLOBOSA (GLO) 27!, SQUAMOSA (SQUA) 10!, and PLENA (PLE) 4! in Antirrhinum majus. Mutations in an AG or PLE gene resulted in homeotic alterations of stamen and carpel. Genetic studies have shown that DEFA, GLO and AP3 genes are essential for petal and stamen development. AP1 and SOUA genes which are expressed in young flower primordia are necessary for transition of an inflorescence meristem into a floral meristem. Sequence analysis of these genes revealed that the gene products contain a conserved MADS box region 4, 10, 11, 15, 25, 27, 30! which is probably a DNA-binding domain 24!. Using these clones as probes, MADS box genes have also been isolated from other species including tomato 17!, tobacco 12!, petunia 2!, Brassica napus 14!, and maize 23!.
Transgenic approaches were undertaken to study the functional roles of the MADS box genes. Genetic complementation of the ag-2 mutant by the AG gene demonstrated that the gene product is involved in stamen and carpel development 30!. Ectopic expression of the AG genes from A. thaliana, B. napus, petunia, tobacco, and tomato resulted in homeotic conversion of sepal to carpel and petal to stamen, mirroring the ap2 mutant phenotype 12, 14, 16, 19, 28!. These results support the hypothesis that AG and AP2 act in an antagonistic fashion. Antisense approaches were also used to reveal the functional role of the tomato MADS box genes 18, 19!. Transgenic plants that express tomato AG antisense RNA displayed the ag mutant phenotypes. Antisense expression of the tomato TM5 MADS box gene resulted in morphological changes in the three inner whorls of transgenic plants.